I'd suggest that it's not so much any colour is "unseeable" as it is "indistinguishable" for some or most people compared to similar colours. And since our perception of colour is relative to different contexts, that makes the research even harder to account for. That all said, an "unseeable" colour to my mind really refers to how if you put two very, very similar colours together, only some people can tell them apart, and perhaps more often once they've learned to recognize the difference? No idea how close I am to the mark, as I haven't listened to any of this or read much source material. But I would compare it to colourblindness -- just because in a certain context someone can't tell red from green doesn't mean it can't be objectively, absolutely measured as such. Which is to say, just because you can't perceive it yourself doesn't mean it can't be studied: either the perception (false or true) or the absolute measurements of reality.